1. Field of the Invention
This invention relates to mode suppression in optical fibers and, more particularly, to the suppression of higher-order modes by resonant coupling in bend-compensated fibers, including, for example, large-mode-area (LMA) fibers.
2. Discussion of the Related Art
Resonant coupling, including index-matching, is a technique for suppressing higher order modes in an optical fiber. [See, for example, J. M. Fini, “Design of solid and microstructure fibers for suppression of higher-order modes,” Optics Express, Vol. 13, p. 3477 (2005), which is incorporated herein by reference.] A straight fiber 10 that relies on resonant coupling, as shown in the material refractive index profile of FIG. 1, includes a core region 12 and a cladding region 14 configured so that radiation propagates in the core region in a fundamental transverse mode 16 and, disadvantageously, in at least one higher-order transverse mode 18. To effect mode suppression the fiber also includes a raised-index cladding feature 14.1 that extends along the length of the fiber. The feature may be a ring (or annulus) encompassing the core or an inclusion, such as a rod (round or otherwise), in the cladding region.
In FIG. 1 the two transverse modes guided in the core region 12 are depicted as a Gaussian-like fundamental mode 16 and the higher-order LP11 mode 18, whereas the transverse mode guided in the feature 14.1 is depicted as a fundamental mode 20. The vertical placement of the modes in this schematic is indicative of the effective indices of the modes. If the mode 20 supported by the feature 14.1 is essentially index-matched with a mode in the core, such as the LP11 higher-order mode (HOM) 18, light will couple from that HOM mode 18 of the core region to the mode 20 of the feature and ultimately to the outer cladding. In preferred designs, the fundamental mode 16 of the core region will not be index-matched to any mode, including specifically mode 20, of the feature. Therefore, the fundamental mode 16 will remain well confined (and propagate with low loss), without coupling to the mode of the feature. In this way, the HOMs are effectively pulled out of the core, where they suffer losses and reduced gain; that is, they are suppressed.
However, in this simple analysis the impact of bending the fiber on index-matched coupling has been neglected. In FIG. 2, the effect of bending the fiber is treated as a tilting of the material index profile 22, which distorts the effective indices of the modes and results in different coupling from the unbent (straight) fiber case; that is, bend-induced distortion produces an equivalent index profile 24 quite different from the designed material index profile 22. Consequently, the bent fiber 10′ may frustrate the intended result. Resonant coupling in the bent fiber 10′ may cause the fundamental mode 16 to be undesirably coupled to the cladding feature mode 20 (with attendant high losses) and may even prevent HOM 18 from being to coupled to mode 20 (with attendant failure of suppression).
Thus, a need remains in the art for a resonant coupling technique that is effective in suppressing HOMs, while maintaining propagation of the fundamental mode, when the fiber is bent.
This need is particularly acute in LMA fibers, which are commonly used in applications that incorporate optical fiber amplifiers and lasers. (In LMA fibers the effective core area illustratively ranges from about 100 μm2 to about 1500 μm2, but could range even higher.) In these applications, the relatively large core area of a LMA fiber accommodates higher power but also permits more HOMs to propagate. Strategies exist for suppressing HOMs in straight fibers, but in typical amplifier or laser packages the LMA fiber is coiled so that it fits within the package space. Under such circumstances, the radius of the coiled fiber (or at least a range of radii) is known a priori, a fact which is exploited in a preferred embodiment of my invention.